The invention relates to a method for forming a field emission cold cathode, and more particularly to a method for forming a cone-shaped field emission cold cathode having a top which is sharply pointed for improvements of emission properties.
Normally, a field emission electron gun 10 has a structure as illustrated in FIG. 1. The field emission electron gun 10 has a plurality of cone-shaped field emission cold cathodes 12 which are formed on a single silicon substrate 11. Each the cone-shaped field emission cold cathodes 12 is made of a refractory metal such as tungsten, molybdenum, tantalum and niobium. An insulation film 13 made of silicon oxide is formed on the silicon substrate 11. The insulation film 14 has a plurality of cavities 13, each of which accommodates each of the cone-shaped field emission cold cathodes 12. Gate electrodes 15 made of a refractory metal such as tungsten, molybdenum, tantalum and niobium are formed on the insulation film 14. The gate electrodes 15 have the same level as the top of each of the cone-shaped field emission cold cathodes 12. The gate electrode 15 encompasses and is separated from the top portion of each the cone-shaped field emission cold cathode 12.
A bias is applied at a few voltages between the gate electrode 15 and the cone-shaped field emission cold cathode 12 so as to cause electron emission from the top of each the cone-shaped field emission cold cathodes 12 without heating the cathodes 12.
A conventional method for forming a cone-shaped field emission cold cathode of Spindt type will be described with reference to FIGS. 2A through 2E. As illustrated in FIG. 2A, a silicon oxide film 14A having a thickness of 1 micrometers is formed on a silicon substrate 11 by a chemical vapor deposition. A gate layer 15A is deposited on the silicon oxide film 14A. The gate layer 15A has a thickness of 0.4 micrometers and is made of a refractory metal such as tungsten, molybdenum, tantalum and niobium. A photo-resist film 16 is applied on the gate layer 15A.
As illustrated in FIG. 2B, holes 16a having a diameter of 1 micrometer are found in the photo-resist film 16 at a pitch of 10 micrometers. The gate layer 15A and the silicon oxide film 14A are selectively etched using the photo-resist film 16 to thereby form an insulator 14 having a cavity 13 and a gate electrode 15. The photo-resist film 16 used is then removed.
As illustrated in FIG. 2C, the silicon substrate 11 is rotated in a plane parallel to the surface of the silicon substrate 11. Aluminum atoms are deposited at an angle of approximately 15 degrees to the surface of the silicon substrate 11 to form an aluminum film 17 having a thickness of 0.15 micrometers. The aluminum film 17 extends toward the center of the cavity 13 to make the opening 15a of the gate 15 narrow.
As illustrated in FIG. 2D, refractory metal atoms such as tungsten, molybdenum, tantalum and niobium are deposited in a vertical direction to the surface of the silicon substrate 11 whereby a refractory metal cone 12 is formed in the cavity 13 and further a refractory metal layer 18 is deposited on the aluminum film 17. The refractory metal layer 18 has a concave portion being cone-shaped and posited over the opening 15a.
As illustrated in FIG. 2E, the refractory metal layer 18 and the aluminum film 17 are removed to expose the gate electrode 15 on the silicon oxide film 14 and the refractory metal cone 12 within the cavity 13. The refractory metal cone 12 serves as a cone-shaped cathode.
According to the above conventional method, the cone 12 is formed by the vertical deposition of refractory metal atoms while the aluminum film 17 is formed by the deposition of aluminum at the oblique angle. The deposition at the oblique angle of aluminum on the gate electrode 15 tends to cause a small variation in the shape and the position of the opening edge of the aluminum film 17. The shape of the cone 12, however, depends upon the shape and the position of the opening edge of the aluminum film 17. Any small variation in the shape or the position of the opening edge of the aluminum film 17 results in a considerable variation in the shape of the cone 12, particularly the top shape thereof. For the emission properties, the top shape of the cone 12 serving as the cathode is extremely influential. Any slight deformation of the top of the cone 12 results in a considerable deterioration of the electron emission properties. This may result in a reduction in the yield of the field emission electron gun having the cone-shaped field emission cold cathode. In order to obtain stable and desirable electron emission properties, it would be essentially to form a field emission cold cathode having a top sharply pointed without any deformation. For this reason, if the above conventional method is used for forming the field emission cold cathode, then it is difficult to obtain the desired top shape which is pointed without any deformation.
There is another method for forming a field emission cold cathode of Gray type which is disclosed in the U.S. Pat. Nos. 4,307,507 and 4,513,308. A silicon oxide film is formed on a silicon substrate and then pattered to thereby form a silicon oxide pattern on the silicon substrate. The silicon substrate is then subjected to an anisotrophy etching by using the silicon oxide pattern as a mask to form a cone on the silicon substrate. A surface of the cone is subjected to both oxidation and subsequent lift-off using a fluorine acid to thereby form a sharply pointed top of the cone which serves as a field emission cold cathode.
As described above, the oxide film which coats the refractory metal cone is selectively etched using the lift-off process using a fluorine acid so that the top portion of the refractory cone is exposed. If the lift-off process is used with a fluorine acid to selectively remove the oxide film covering the top of the cone then the top of the refractory metal cone is likely deformed from the desirable top shape which is sharply pointed. Any small variation in the shape or the position of the opening edge of the aluminum film 17 results in a considerable variation in the shape of the cone 12, particularly the top shape thereof. For the emission properties, the top shape of the cone 12 serving as a cathode is extremely influential. Any slight deformation of the top of the cone results in a considerable deterioration of the electron emission properties. This may result in a reduction in the yield of the field emission electron gun having the cone-shaped field emission cold cathode. In order to obtain stable and desirable electron emission properties, it would be essential to form a field emission cold cathode having a top sharply pointed without any deformation. For this reason, if the above conventional method is used for forming the field emission cold cathode, then it is difficult to obtain the desired top shape which is pointed without any deformation.